Negative return codes not being handled

[saludovencedores]

I'm new to this world and not the programmer who implemented our donation page, but our implementation is allowing transactions to be processed where the address of the donor does not match the address of the card owner, the CVV code given by the donor does not match the card's, and even the name of the donor does not match the name of the card owner.

Our donate page is located here: https://donate-africawaterandlife.appspot.com/

Would you please take a look at this page and tell me (in hopefully fairly simple terminology) what we're doing wrong and how to correct this problem?

Thank you.